The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at quick downshifting for rapid acceleration, such as kickdown.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio.
When command of rapid acceleration of the vehicle, which is dependent on driver's operation of an accelerator pedal, is inputted in the control system, it is necessary to downshift the transmission or to control the transmission ratio to meet the requirement.
Various systems for controlling the transmission ratio at transient state are disclosed. Japanese Patent Application Laid-Open No. 59-208253 discloses a system wherein a transmission ratio is fixed to a predetermined value at an initial stage of acceleration. Further, Japanese Patent Application Laid-Open No. 60-88259 discloses a system which determines changing speed of desired transmission ratio in dependency on the operating speed of an accelerator pedal.
However, in accordance with the system of the prior art, the actual transmission ratio does not change to provide rapid acceleration of the vehicle at quick depression of an accelerator pedal so as to meet driver's demand.